In a catalyst apparatus for purifying an exhaust gas discharged from an internal-combustion engine of an automobile or the like, various catalysts have been used depending on object thereof. As a major catalyst component thereof, there is included platinum group metals, and usually it is used by loading, in high dispersion, onto a refractory inorganic oxide having high surface area of activated alumina or the like (refer to PATENT LITERATURE 1).
As the platinum group metals as the catalyst component, there have been known platinum (Pt), palladium (Pd), and rhodium (Rh), which have been used widely as the catalyst for purifying exhaust gas discharged from an internal-combustion engine of an automobile or the like. In the aforementioned TWC, a catalytically active species superior in oxidation activity, of Pt, Pd or the like, and Rh superior in purification activity of NOx, are used in combination, in many cases. In recent years, regulations on hazardous substances contained in exhaust gas, in particular, regulations on NOx, have become more and more severe. Accordingly, it is necessary to effectively use Rh superior in purification activity of NOx. In addition, Rh is scarce in production and of high price, which has caused price hike in recent market. Therefore, it is preferable to decrease used amount of Rh as a catalytically active species, in view of resource protection as well as cost.
In addition, in the catalyst for purifying exhaust gas, in order to achieve the enhancement of still more purification performance, the addition of various promoter components, other than platinum group metals, to the catalyst has been investigated. As these promoter components, an Oxygen Storage Component (OSC), or an alkaline earth metal, or zirconium oxide, zeolite and the like have been known.
Among these, OSC stores and releases oxygen in exhaust gas, and cerium oxide has been known as an OSC. Cerium oxide stores oxygen as CeO2, when oxygen concentration is high in exhaust gas, and releases oxygen by being converted to Ce2O3, when oxygen concentration is low. Oxygen released is active and it promotes purification of HC and CO, by being utilized in oxidation action by Pt or Pd. In addition, OSC also serves to buffer oxygen concentration change in exhaust gas, by storage and discharge of oxygen. By this action, purification performance of exhaust gas is enhanced in TWC. TWC is a catalyst which performs both oxidation and reduction, and has a range of exhaust gas components suitable for purification, in view of designing. This range depends on air/fuel ratio in many cases. Such a range is called a window, and in many cases, exhaust gas combusted at the neighborhood of theoretical air-fuel ratio, so-called stoichiometric ratio, is set as the window region. By being buffered change in oxygen concentration in exhaust gas, this window region can be maintained for a long period of time, and purification of exhaust gas is performed effectively. This is said to influence particularly on purification characteristics of NOx by Rh.
As such a cerium oxide, although a pure cerium oxide can be used, a composite oxide with zirconium is often used (refer to PATENT LITERATURE 2). A cerium-zirconium composite oxide is said to have high heat resistance and high storage and release rate of oxygen. It is believed because a crystal structure of the cerium-zirconium composite oxide is stable, and it does not inhibit the action of a cerium oxide, which is a main OSC component, thus all parts including inside of the particle can be utilized for action as the OSC.
On the other hand, in purification of NOx by Rh, it is considered that, for example, a steam reforming reaction or a (CO+NO) reaction is promoted via the Rh component as shown below and to purify NOx.HC+H2O-----→COx+H2  (1)H2+NOx-----→N2+H2O  (2)CO+NO-----→CO2+½N2  (3)
And, it is known that the zirconium oxide promotes the steam reforming reaction and the (CO+NO) reaction, when used together with the Rh component (refer to PATENT LITERATURE 3).
As other promoter component, an alkaline earth metal such as the Ba component has also been known (refer to PATENT LITERATURE 4). The Ba component is converted to Ba(NO3)2 by temporarily storing NOx contained in exhaust gas, and after that purifies the stored NOx by reducing to N2 by a reducing component contained in exhaust gas.
In general, NOx is generated in a large quantity, when fuel supplied to an engine is less, and amount of air is more, and combustion temperature is high. The Ba component thus temporarily absorbs NOx generated.
NOx absorbed onto the Ba component is discharged from the Ba component when NOx concentration in exhaust gas becomes low and carbon dioxide (CO2) concentration becomes high. This is caused by reaction of the above-mentioned Ba(NO3)2 with carbon dioxide gas under co-existence of steam, to be converted to BaCO3, which occurs towards the chemical equilibrium. NOx discharged from the Ba component, as described above, is purified by reduction, by reacting with a reducing component at the Rh component surface.
Such a promoter component can also be used in combination of two or more materials and, for example, TWC has been known where the Ba component and cerium oxide are used (refer to PATENT LITERATURE 5). However, purification performance can be decreased depending on combination of catalyst materials: for example, it has been reported that presence of the Rh component and the Ba component in the same composition decreases NOx purification performance (refer to PATENT LITERATURE 6). It is believed that this is because the alkaline earth metal component has action of storing NOx, purification action of NO in the Rh component is interfered, and/or an oxidation Rh structure is stabilized by electron donating action from Ba to Rh.
Therefore, it has been proposed to enhance NOx purification performance and heat resistance, by loading the Rh component and the Ba component onto alumina in a separated state (refer to PATENT LITERATURE 7). In this Literature, there is no description on what degree the Rh component and the Ba component are separated in the catalyst layer. If water-soluble Ba acetate is used as a Ba source, the Ba component dissolves into slurry, and it is not likely that Ba is sufficiently separated from the Rh component. As a result, the Rh component and the Ba component come close, and a problem of decrease in NOx purification performance will not be solved sufficiently.
Thus, there are various combinations of the catalyst components, and complicated reaction routes are taken by interaction of the catalyst components, and thus by overall investigation on these, researchers have been searching a combination of the catalyst components which exerts purification action most.
By the way, the catalyst for purifying exhaust gas may be arranged just one in exhaust gas passage, however, there may be the case where two or more pieces are arranged. This aims at more utilization of characteristics of the catalyst for purifying exhaust gas, in association with stricter exhaust gas regulations, and each optimum position is determined according to durabilities (heat resistance, atmosphere resistance, and poisoning resistance), catalyst characteristics (oxidation activity and reduction activity) or the like which each noble metal of platinum, palladium and Rhodium has.
Also, in order to reduce use of noble metals or rare earth metals leads and to efficiently use limited resources, it has been required to install the catalyst for purifying exhaust gas at the optimum position of exhaust gas passage according to characteristics of each noble metals or rare earth metals.
Still more, regulations on exhaust gas has become more and more severe in recent years, and advent of such a catalyst has been desired that exerts more superior purification performance of exhaust gas, by using a plurality of catalysts. Among the regulations for exhaust gas, that of NOx has become particularly severer, and thus also in TWC, needs of the catalyst for purifying exhaust gas superior in NOx purification performance has increased.